db: Add virtual tables interface

This change introduces the basic interface we expect each virtual
table to implement. More specific implementations will then expand
upon it if needed.

CMakeLists.txt

@@ -434,6 +434,7 @@ set(scylla_sources
     db/sstables-format-selector.cc
     db/system_distributed_keyspace.cc
     db/system_keyspace.cc
+    db/virtual_table.cc
     db/view/row_locking.cc
     db/view/view.cc
     db/view/view_update_generator.cc

configure.py

@@ -841,6 +841,7 @@ scylla_core = (['database.cc',
                 'cql3/variable_specifications.cc',
                 'db/consistency_level.cc',
                 'db/system_keyspace.cc',
+                'db/virtual_table.cc',
                 'db/system_distributed_keyspace.cc',
                 'db/size_estimates_virtual_reader.cc',
                 'db/schema_tables.cc',

db/virtual_table.cc

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * Modified by ScyllaDB
+ * Copyright (C) 2021 ScyllaDB
+ */
+
+/*
+ * This file is part of Scylla.
+ *
+ * Scylla is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Scylla is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "db/virtual_table.hh"
+
+namespace db {
+
+void virtual_table::set_cell(row& cr, const bytes& column_name, data_value value) {
+    auto ts = api::new_timestamp();
+    auto cdef = schema()->get_column_definition(column_name);
+    if (!cdef) {
+        throw_with_backtrace<std::runtime_error>(format("column not found: {}", column_name));
+    }
+    if (!value.is_null()) {
+        cr.apply(*cdef, atomic_cell::make_live(*cdef->type, ts, value.serialize_nonnull()));
+    }
+}
+
+bool virtual_table::this_shard_owns(const dht::decorated_key& dk) const {
+    return dht::shard_of(*_s, dk.token()) == this_shard_id();
+}
+
+bool virtual_table::contains_key(const dht::partition_range& pr, const dht::decorated_key& dk) const {
+    return pr.contains(dk, dht::ring_position_comparator(*_s));
+}
+
+}

db/virtual_table.hh

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2020 ScyllaDB
+ */
+
+/*
+ * This file is part of Scylla.
+ *
+ * Scylla is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Scylla is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "mutation_reader.hh"
+#include "schema.hh"
+#include "database_fwd.hh"
+
+namespace db {
+
+class virtual_table {
+protected:
+    schema_ptr _s;
+    database* _db = nullptr; // Always valid when attached to a database.
+
+protected: // opt-ins
+    // If set to true, the implementation ensures that produced data
+    // only contains partitions owned by the current shard.
+    // Implementations can do this by checking the result of this_shard_owns().
+    // If set to false, data will be filtered out automatically.
+    bool _shard_aware = false;
+
+protected:
+    void set_cell(row&, const bytes& column_name, data_value);
+    bool contains_key(const dht::partition_range&, const dht::decorated_key&) const;
+    bool this_shard_owns(const dht::decorated_key&) const;
+
+public:
+    explicit virtual_table(schema_ptr s) : _s(std::move(s)) {}
+
+    const schema_ptr& schema() const { return _s; }
+
+    // Keep this object alive as long as the returned mutation_source is alive.
+    virtual mutation_source as_mutation_source() = 0;
+
+    void set_database(database& db) { _db = &db; }
+};
+
+}